Today's telecommunication networks are characterized by specialization. This means that for every individual telecommunication service, at least one network exists that transports this service. Each of these networks was specially designed for that specific service and is often not applicable to transport another service. For example, advances in audio, video, and speech coding in compression algorithms in VLSI technology influenced the bit rate generated by a service, and thus changed the service requirements for the network. In addition, resources that are available in one network, generally are not available for other networks. Thus, the need for a service-independent network is evident based upon these disadvantages from existing networks.
Asynchronous Transfer Mode (ATM) is a communication protocol for use in a service independent network. With ATM, information for multiple service types, such as voice, video or data is conveyed in small, fixed-size cells. ATM technology combines the benefits of circuit switching (guaranteed capacity and constant transmission delay) with the benefits of packet switching (flexibility and efficiency for intermittent traffic). ATM technology provides a scalable bandwidth from a few megabits per second (Mbps) to many gigabits per second (Gbps). Because of its asynchronous nature, ATM is more efficient than synchronous technologies, such as time-division multiplexing (TDM).
ATM transfers information in fixed-sized units called cells. Each cell contains 53 bytes. FIG. 1 is a schematic of a standard ATM cell 10 format as defined by the ATM Forum and the International Telecommunications Union's Telecommunication Standardization Sector (ITU-T). ATM cell 10 contains a five-byte header 11, which contains all of the information necessary for network management. ATM cell 10 also contains a payload 13, which is the remaining 48 bytes.
ATM header 11 has a different format depending on whether it is UNI or NNI. The ATM header 11 of FIG. 1 is configured for UNI. For byte 1, bits 8-5 correspond to the Generic Flow Control (GFC) field. The GFC field allows a multiplexer to control the rate of an ATM terminal. The GFC can be used to provide local functions, such as flow control at the UNI. These bits are not carried end-to-end and may be overwritten by local switches. However, the GFC field is typically not used and is set to a default value.
The next 8 bits (bits 4-1 of byte 1 and bits 8-5 of byte 2) correspond to the Virtual Path Identifier (VPI) field. VPI is used in conjunction with the Virtual Channel Identifier (VCI), to identify the next destination of a cell as it passes through a series of ATM switches on the way to its destination. The next 16 bits of ATM header 11 represent the VCI. Like the VPI, the VCI is used to identify the next destination of a cell as it passes through a series of ATM switches on the way to its destination.
The next 3 bits (bits 4-2 of byte 4) correspond to the Payload Type Indicator (PTI) field. The first PTI bit indicates whether the ATM cell 10 contains user data or control data. If the cell 10 contains user data, the second PTI bit indicates congestion, and the third PTI bit indicates ATM user to ATM user information, typically used to indicate the end of an ATM adaptation layer 5 (AAL5) frame. A single bit following the PTI corresponds to the Cell Loss Priority (CLP). CLP bit indicates whether the cell should be discarded if it encounters congestion as it moves through the network. If the CLP bit equals one, then the cell should be discarded in preference to cells with the CLP bit equal to zero. The final 8 bits, which are in byte 5, correspond to the Header Error Control (HEC). The HEC bits are a checksum calculated on the header 11 itself.
The NNI header format varies slightly from the UNI header format described above. The difference lies in the fact that the GFC field, as discussed above in the UNI header format 11, is not present in the NM header format. Instead, the VPI field occupies the first 12 bits of header 11. This allows the ATM switches to assign larger VPI values. With that sole exception, the NNI header is identical to the format of the UNI header 11.
An ATM switch generally has several inputs and several outputs that are carrying ATM data. The information in the header 11 carries the navigational information for each ATM cell 10 so that the ATM cell 10 is provided to the proper output. Each input and output can be said to be a physical layer device. The ATM switch includes resources that will process each cell 10 at a higher layer from the physical layer devices, the ATM layer. Once processed, each ATM cell 10 can be provided to the proper output physical layer device. The different layers are in reference to the International Standards Organization's Open Systems Interconnection (ISO-OSI) international standard. A data bus is typically used to transport the ATM cells from the ATM layer to several different physical layer devices. The Universal Test & Operations PHY Interface for ATM (UTOPIA) data path interface has become a standard for the data bus that transports the ATM cells. The current UTOPIA data path interface, or UTOPIA bus is typically only used for standard ATM cell transfer.
Data that is not processed into cells cannot be transferred via the UTOPIA bus. Processing the data into standard ATM cells might require the data to be carried through the entire ATM network, which may not be desirable. Instead separate data buses for each different type of data may be required. Each bus specifically designed for each type of data. For example, digitized voice data such as pulse code modulation (PCM) data typically requires a time division multiplexed (TDM) enabled bus. Another example is packet data, such as Ethernet data, which may require a specially designed bus, such as the Media Independent Interface (MII) bus. From a system design point of view, separate data buses may be very expensive and complicated. It would be desirable to transfer data of various data types across a single data bus, for example, the UTOPIA bus, so as to eliminate the need for multiple
buses, without encapsulating packet and/or voice data into a standard ATM cell.
Accordingly, there is a need for improved methods and systems for transferring data of various non-ATM data types across a common data path interface without encapsulating packet data and/or voice data into standard ATM cells.